This invention relates to the field of heating units, especially heating units intended for use in processes requiring precisely-controlled amounts of heat.
One conventional method of producing a measured amount of heat is to activate or deactivate a heater according to the output of an independent temperature sensor. The temperature sensor could be a liquid or gas bulb, a thermocouple, a thermistor, an RTD sensor, or other equivalent device. The major disadvantage of this method is that there is an inherent delay between the initial sensing of temperature, and the response by the control device which activates or deactivates the heater. In addition to the problem of delay, a control device using a separate temperature sensor usually causes the heater to overshoot or undershoot its target temperature.
In theory, by combining the heat source and the temperature sensor into a single unit, one can eliminate sensor lag, and can therefore construct electric heating systems which have extremely fast response times, and which are also highly accurate. Such properties are especially important in the construction of high-performance process heating systems, because the thermal mass, and therefore the physical size, of the heating system can then be substantially reduced, and the energy efficiency of the system can be improved. Also, a unit comprising a combination of a heat source and temperature sensor can be tailored to match precisely the needs of the process under all operating conditions.
U.S. Pat. No. 3,679,871 discloses an example of a unit comprising both a heat source and a temperature sensor. This patent, the disclosure of which is incorporated by reference herein, shows a circuit which includes a Wheatstone bridge. One arm of the bridge is a resistive heating element having a positive temperature coefficient. When heat is conducted away from the vicinity of the heating element, the heating element cools, and its electrical resistance decreases, thereby unbalancing the bridge. Through appropriate electronic circuitry, the condition of unbalance causes a relatively large heating current to flow through the heating element, causing the latter to generate more heat, until the resistance of the heating element again reaches the point where the bridge is in balance. Thus, the temperature sensor is the same as the heating element: the circuit does not directly measure temperature, but rather "sees" only the electrical resistance of the heating element.
The above-described circuit is functional, and represents a great improvement over systems using separate temperature sensors and heating elements. But the circuit still has substantial disadvantages. The circuit lacks sensitivity, is limited in range, and exhibits large amounts of "drift". It also generates substantial electrical noise, and produces unacceptably high thermal emissions as well. These deficiencies have substantially impeded the commercial potential of the circuit.
The present invention provides an improvement over the combined heating element and temperature sensor described above. The invention provides an economical but highly accurate and sensitive control device for maintaining a desired amount of heat in an industrial or other process.